Process for preparing photographic emulsions



United States Patent 3,153,594 FRQQESS EUR PREPARING EHUTQGFl-HC EMULSIQNS Adolf Eduard Oberth, Granger/ale, Caiih, assignor to E. l. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware No Drawing. Filed July 7, 1959, Ser. No. 825,417 8 Qlaims. (ill. 96-414) This invention relates to a novel process for preparing photographic silver halide emulsions. More particularly, it relates to a process for concentrating a photographic silver halide emulsion by coagulating it following the digestion (or second ripening) step.

The coagulation of ripened silver halide photographic emulsions and washing of the coagulated emulsions is known. Thus, silver halide grains have been ripened in the presence of very small amounts of gelatin or other water-permeable protective colloids, coagulated along with the protective colloid by various agents (e.g., organic solvents, acids, and anion soaps), redispersed in the presence of larger amounts of a water-permeable protective colloid, chemical sensitized (normally at elevated temperatures), and finally, coated and dried in the normal manner.

An object of this invention is to provide a process for preparing coagulated, washed photographic silver halide emulsions of high solids content and enhanced sensitometric properties. Another object is to provide such a process which utilizes the conventional apparatus of photographic manufacturers. A further object is to provide such a process which utilizes available chemicals and gives uniform, dependable results. Still further objects will be apparent from the following detailed description of the invention.

In its broader aspects, the process of this invention comprises (a) redispersing a washed coagulum of a gelatino-silver halide emulsion in an aqueous solution so that not more than about 25% by weight of gelatin, based on the silver halide, is present in the redispersion, (b) adding a chemical sensitizing agent and digesting the redispersion at a temperature of at least 90 F., (c) coagulating the redispersion, (d) removing the supernatant liquid and (e) redispersing the recoagulated dispersion in an aqueous solution containing a water-permeable colloid of high molecular weight and possessing protective cololid properties. If desired, in carrying out the process after removal of the supernatant liquid in step (d) the coagulum is washed with water.

The coagulum used in step (a) can be made in any of the conventional manners by admixing with a precipitated and ripened gelatino-silver halide emulsion a coagulating agent, e.g., acetone, dimethyl sulfate, an anion soap or a water-soluble, acid-soluble polymer of high molecular Weight (e.g., 5,000 to 50,000 or more) containing a plurality of acid groups, e.g., carboxylic acid groups, sulfate groups or phosphate groups, or the alkali metal or ammonium salts of such groups. Suitable anion soaps and poly electrolytes are described in US. patents Waller et al. 2,489,341 and Moede 2,772,165, respectively.

The foregoing types of coagulating agents can be used in recoagulation step (c). However, since the emulsions already contain some coagulating agent, recoagulation can be attained by adding an acid which does not have an adverse eifect on photographic emulsions, e.g., acetic, to reduce the pH below 6.0.

During step (b) any of the conventional adjuvants for photographic emulsions can be added to the redispersion. Suitable such adjuvants include optical sensitizers, halides for controlling pAg, stabilizers and anti-fogging compounds, acids or alkalies for pH adjustment, etc.

Prior to step (e), the recoagulated dispersion can be 3,153,504 Patented Got. 20, E964- "ice stored for any reasonable length of time. When it becomes convenient to coat the emulsion, it is redispersed in the desired binder, which can be gelatin or any other compatible water-permeable organic colloid binding agent, by raising the pH above 6.0 and stirring at about the desired coating temperature. During or following the redispersion the emulsion is diluted with water to Whatever concentration required for the type and thickness of the coating to be made. At this time any of the conventional adjuvants added to emulsions after digestion can be admixed with the redispersion and the redispersion coated onto any suitable support.

The invention will be further illustrated by, but is not intended to be limited to, the following detailed examples wherein the emulsions were made and coated under safelight conditions.

Example I A high-speed gelatino-silver iodobromide emulsion containing 3 moles of silver halide (1.7 mole percent Agl and 98.3 mole percent AgBr) and 40 g. gelatin was precipitated and ripened in a conventional manner. An aqueous solution of a mixture of sulfated alcohols, predominantly sodium dodecyl sulfate, was then added to give a gelatin/ sulfate ratio of 1:069. The emulsion was stirred and ml. of acetic acid added to form a coagulum containing the suspended silver halide grains. Supernatant liquid was decanted and the coagulum washed twice with solutions containing 5000 ml. H 0, 50 g. NaCl, and 10 ml. acetic acid.

A quantity of the precipitated emulsion containing 1.5 moles of silver halide and with approximately 10 g. of gelatin remaining after the washing, was added to an aqueous gelatin solution containing 30 g. of gelatin in 2400 ml. of water. On adjusting the pH to 6.3 with NaOH and stirring at F., the coagulate was redispersed. Under constant stirring to avoid settling of the suspended matter, gold, sulfur, and reduction sensitizers were added in the normal manner. The emulsion was then heated to 140 F., digested at this temperature for 40 minutes, and cooled to F. Recoagulation was effected by the addition of 300 ml. of a 5% aqueous solution of a mixture of sulfated alcohols, predominantly sodium dodecyl sulfate, followed by the addition of a solution made up of 300 ml. H 0, 70 g. NaCl, and 4 ml. glacial acidic acid. After settling for 10 minutes the supernatant liquid was decanted and the coagulate was washed with 2000 ml. of slightly acidified cold water.

Three portions were taken from this emulsion coagulate, each portion containing O.l5 moles of silver halide. To each of these emulsion portions, labelled A, B, and C, were added 27 g. of one of the following gelatin types, as designated:

Avery inert gelatin.

B-relatively inert gelatin.

C-pigskin gelatin (relatively active, with tendency to cause fog).

The three emulsions were redispersed in the gelatin solution as described above by adjusting the pH between 6.4 and 6.8 and stirring vigorously. After addition of the usual post-digestive adjuvants, e.g., antifoggants, wetting agents, halide solutions, etc., the emulsions were coated and dried in the usual manner on cellulose triacetate photographic film base to give coating Weights of of approximately milligrams of silver halide per square decimeter.

Film strips from each of these coatings were exposed in a sensitometer equipped with a neutral density squareroot-of-two step wedgeaccording to a procedure based on the American Standard Method for the Sensitometry of Medical X-Rray Films--PH 2.9l956.' After exposure,

the film strips were developed for 5 minutes at 68 F. in a solution of the following composition:

G. Metol 3.0 Hydroquinone 9.0 Na S anhydrous 50.0 K 00 50.0 KBr. 4.5

15 0 to make 1 liter.

After fixing and drying the film strips their densities were read on a Western Electric RA-l 100C Densitometer, giving the following results:

Gelatin Type Speed Fog Step No. 16,

Total Density A 0.60 0. 06 1.73 B 0. 61 06 1. 86 C 0. 45 .05 1. 54

*Specd is given as the direct reading of optical density at Step No.11 of the wedge exposure with subtraction of the density due to fog.

In this example over 90% of the gelatin present in the finished product was added during the final redispersion which occurred just prior to coating. This decreases the dependency of the emulsion upon the particular type of bulk gelatin employed and makes possible the use of a wide variety of gelatin types.

Example II A cine positive silver iodobromide emulsion containing 10 g. of gelatin and approximately 3.6% silver iodide and 96.4% silver bromide based on the total weight of silver halides was made by precipitating the silver halides in a 0.59% aqueous gelatin solution in an amount to give a ratio of gelatin to silver halide of about 1:9. The emulsion was ripened at 120 F. for 17 minutes.

To this emulsion which amounted to about 740 grams including the water there was added a solution made up of 100 ml. of distilled water, 10 m1. of glacial acetic acid, and 12 ml. of an 8.8% solution of a soluble partial acetal of polyvinyl alcohol and o-sulfobanzaldehyde consisting of grams of sulfonate sulfur per 100 grams of polymer. The resulting coagulurn was allowed to settle in the manner described in Moede US. Patent 2,772,165. After removing the supernatant liquid by decantation the curds were washed twice with 8 liters of distilled water to remove the soluble salts. Five hundred grams of the washed coagulate were redispersed by stirring in a solution of 30 grams of gelatin in 2400 ml. of distilled water by adjusting the pH to 6.3 and the temperature to 95 F. Gold and sulfur sensitizers in excess were then added and the emulsion was digested for 30 minutes at 145 F. The emulsion was cooled to 81 F. and the pH was lowered to 4.5 with 25 ml. of acetic acid solution. This brought about the coagulation of the suspended matter without addition of any other coagulating agent. After decanting the supernatant liquid the resulting coagulate was washed once with distilled Water to remove excess sensitizers. To the precipitated emulsion, free of supernatant liquid, there were added 400 ml. of a 5% aqueous solution of sodium dodecyl sulfate. This addition was for the purpose of making the gelatin compatible with polyvinyl alcohol as disclosed in Oberth et al. US Serial No. 803,356, filed April 1, 1959 (U.S. Patent 3,067,035, December 4, 1962). The pH was adjusted to 6.3 with aqueous sodium hydroxide and the curds were redispersed by stirring for 5 minutes at about 100 F. The total weight of the emulsion was adjusted to 1000 grams with water. Three-hundred gram portions of this emulsion were each mixed with (I) 450 grams of a 15% aqueous gelatin solution and (II) 450 grams of a 15% aqueous solution of deionized high molecular weight polyvinly alcohol. The emulsions after thorough mixing were coated on the film supports described in Cohen and Heiart Ser. No. 783,313, filed December 29, 1958 (U.S. Patent Emulsion Relative Gamma Fog Speed Gelatin Emulsion (Control) 1. 5 05 Polyvinyl Alcohol Emulsion- 1125 1. 9 15 The polyvinyl alcohol emulsion showed much higher resistance to dimensional change, blistering and deterioration under high intensity are light projection.

Example Ill Two and a quarter moles of silver halide grains were formed and the gelatin emulsion ripened as described in Example I. It was then coagulated by addition of acid and the partial acetal of polyvinyl alcohol and o-sulfobenzaldehyde (consisting of 5 grams of sulfonate sulfur per 100 grams of polymer) and washed as described in Example I of Moede, U.S. Patent 2,772,165. The re sultant coagulate was redispersed in a solution of 45 g. of a very inert gelatin in 2600 g. of distilled E 0 by adjusting the pH up to 6.3 and vigorously stirring at 95 F. for 15 minutes. Gold, sulfur, and reduction sensitizers were added in the conventional manner, and the emulsion was digested for 40 minutes at 140 F. After cooling to 95 F., one-third of the emulsion (containing 0.75 mole of silver halide) was separated and labeled A. The remainder of the emulsion was recoagulated by adding ml. of a 50% (weight to volume) solution of NH NO 50 ml. of the 8.8% solution of the partial acetal of polyvinyl alcohol and sodium o-sulfobenzaldehyde described in Example II, and suificient glacial acetic acid to lower the pH to 3.6. The emulsion was cooled in running cold tap water to improve coagulation, the supernatant liquid was removed, and the emulsion was washed for 10 mintues with high shear stirring in 300 ml. of cold, slightly acidified tap water. Next, the emulsion was redispersed by adding 250 g. of distilled Water, adjusting the pH up to 6.3, and stirring. It was then divided in 2 equal portions, each containing 0.75 mole of silver halide and labeled B and C. Portions A and B were bulked up with aqueous solutions containing g. of the same gelatin that was present during digestion. Portion C was bulked up with an aqueous solution containing 125 g. of a relatively more active gelatin.

After addition of the usual post-digestive adjuvants the emulsions were coated in the usual manner on cellulose triacetate photographic film base to give coating weights of approximately 90 milligrams per square decimeter. Film strips of the dried coatings were exposed, processed, and evaluated as described in Example 1 to give the following sensitiometric results:

*See footnote, column 3. Tropical Oven conditions, 120 11, 65% RH, 7 days.

With a coagulation wash step following digestion both sensitivity and fog of aged film are much improved. It is also noted that the control emulsion (A) with the more expensive gelatin (very inert) is less stable on aging than emulsion C which contained a cheaper gelatin but which employed the second coagulation wash step.

Example IV An emulsion which had been ripened, coagulated, washed, redispersed, chemically sensitized, and digested in the manner described in Example ill contained 3 moles I of silver halide. At the end of digestion the emulsion was cooled to 100 F. and coagulated by addition of dilute acetic acid to pH 4.5. After removing the supernatant liquid, one-twentieth of the coagulate (containing 0.15 mole of silver halide) was removed. This portion was redispersed by adding it to an aqueous solution containing 25 g. of gelatin, adjusting pH to 6.2, and stirring at 100 F. The usual post-digestive adjuvants were added and the emulsion was skim coated on cellulose triacetate film base and dried in the usual manner. The usual thin gelatin abrasion coating was applied over the photographic emulsion layer.

The remainder of the coagulate (containing 2.85 moles of silver halide) was easily redispersed by stirring it slowly for 3 minutes at 130 F. in a concentrated aqueous gelatin solution containing 450 g. of gelatin. After addition of the usual post-digestive adjuvants, the emulsion was calculated to contain 50% total solids. Satisfactory extrusion coating and drying of the emulsion was carried out by the process described in Haley, Ser. No. 695,341, filed November 8, 1957, U.S. Patent 3,082,144, March 19, 1963. The coating was made on polyester photographic film base of the type described in Alles et al., 2,627,088, issued February 3, 1953, and Alles, US. 2,779,684, issuedJanuary 29, 1957, both assigned to Du Pont. No abrasion layer was applied over this coat- Film strips from both the skim coating and the extrusion coating were exposed, processed, and evaluated as described in Example I to give the following sensitiometric results:

advantages of this invention, viz., that it is practicable to store highly sensitized photographic emulsion for a considerable period of time after digestion but before coating. It is well known to those skilled in the art that emulsions prepared in the conventional manner cannot be stored more than a few days between chemical sensitization and coating without appreciable loss of sensitivity and/ or in crease in fog.

Age of digested Emulsion at time of coating Speed* Fog *See footnote, col. 3.

Example V1 An emulsion containing 3 moles of silver halide was made, coagulated, and washed as described in Example HI. It was redispersed by stirring in an aqueous solution containing 60 g. of gelatin, the pH of the stirring mixture being adjusted to 6.5. After normal chemical sensitization, the emulsion was digested at 140 F. for 45 minutes and cooled to 100 F. Two portions were removed containing 0.45 mole of silver halide each and the two portions coagulated by the addition of sufficient dilute acetic acid to reduce the pH to 4.5.

After removal of supernatant liquid, the emulsions were redispersed at pH 6.4 by vigorously stirring in aqueous solutions containing about 70 g. of bulk gelatin, different types of gelatin being used for the two emulsions. In one case, a high viscosity, high gel-strength gelatin was added during the redispersion while in the other case a low viscosity, low gel-strength gelatin was used. After addition Coating Step ofthe usual post-digestive adjuvants the emulsions were lXtfi Mg Speed* Fog coated on cellulose triacetate film base by the well known gl y 40 technique of air-knife coating. The coatings were dried and'film strips from these coatings were exposed, proc- Skim Coating 143 1.13 0.07 2.63 essed, and evaluated s escribed in Example I to give Extrusion Coating 146 ihefOllOWing results:

*See footnote, col. 3. Bulk 1 t lttela- Frresh 2A mo. gm. 0

ge a 111 we 0g ge o ontrast Example V speed g Thirty moles of silver halide grains were formed in an emulsion, ripened, coagulated, and washed as described fii g l i nvtiscosity, high gel 110 0 in Example ill. The washed emulsion coagulate was then 50 gg g 3 2- 80 red spersed 111 an aqueous solution containing 600 g. f Strength 100 gelatin by adjusting the pH up to 6.6 and stirring vigorously for 15 minutes at 95 F. Chemical sensitizers were added in the conventional manner, and the emulsion was digested for 45 minutes at 140 F. After digestion, the emulsion was cooled to 100 F., treated with dilute acetic acid to lower the pH to 4.5, and allowed to settle for 15 minutes while cooling to 70 F.

The supernatant liquid was removed by siphoning and a portion of the digested and coagulated emulsion containing 1.5 moles of silver halide were removed. This portion was redispersed in an aqueous solution containing 250 g. of gelatin by raising the pH to 6.2 with dilute NaOH and gently stirring. The usual post-digestive adjuvants were added and the emulsion was coated in the usual manner on cellulose triacetate photographic film base.

The remainder of the digested and coagulated emul sion was stored under refrigeration using an ethanol solution of thymol as a preservative. After 6 days, and again after 51 days, portions of the stored emulsion coagulate (both containing 1.5 moles of silver halide) were redispersed and coated as described in the paragraph above. Film strips of the dried coatings were exposed, processed, and evaluated as described in Example 1. The sensitometric results given below demonstrate one of the unique Example Vll An emulsion was prepared as described in Example III except that digestion was extended to 45 minutes at 140 F. A portion of the emulsion, containing 0.3 mole of silver halide, was coagulated by cooling to F., followed by the addition of dilute acetic acid to a pilot 4.6. After settling for 5 minutes, the supernatant liquid was removed and the emulsion redispersed by stirring'in an aqueous solution containing 50 g. of gelatin and adding sufiicient 1 N NaOH to raise the pH to 6.6.

Another portion of the digested emulsion, also containing 0.3 mole of silver halide, was cooled to 100 F. and to this emulsion was added 50 g. of dry gelatin. After the gelatin was dissolved by stirring, the pH was adjusted to 6.5 with dilute NaOH.

The usual postdigestive adjuvants were added to both J emulsions and they were coated, exposed, and evaluated as described in Example I to give the following results:

*See footnote, col. 3.

This is an obviously over-digested emulsion but it can be seen that fog, particularly after aging, is considerably reduced by the practice of this invention.

Example VIII An emulsion was ripened, coagulated, and washed as in Example III. Two portions of the washed emulsion coagulate were split off, each containing 3 moles of silver halide, and redispersed by raising the pH to 6.3 and stirring in aqueous gelatin solutions containing 50 g. of gelatin in (A) 2100 g. of distilled water and (B) 4800 g. of distilled water. Both emulsions Were sensitized identically with conventional chemical sensitizers, digested for 40 minutes at 140 F., and cooled to 100. The usual post-digestive adjuvants were added and the emulsions were coated, dried, and evaluated as in Example VI to give the following results:

*See footnote, col. 3. Seven days at 120 F., 65% RH.

This example is illustrative of the observation that, as is known to those skilled in the art, emulsions must normally be reasonably dilute during digestion in order to achieve optimum sensitization. In other words, it is not always practical toprepare concentrated emulsions simply by omitting water during their preparation. However, when a concentrated emulsion is desired, this could be very easily and satisfactorily accomplished by the process of this invention, e.g., see Example IV.

Among the useful acids to lower the pH for coagulation are the strong mineral acids, e.g., nitric, sulfuric, and hydrochloride acids. Other acids, such as acetic acid may be used as long as they are photographioally inert and have reasonably high dissociation constants in order to avoid the use of excessive amounts of acid.

Acid alone is satisfactory as a coagulating agent for the second coagulation of certain emulsions such as described in Example IV, in which much of the coagulating agent remained in the emulsion from the first coagulation step. It is frequently advantageous, however, to employ other coagulating agents in addition to the acid, e.g., polymers and coplymers which are both water-soluble and acid-soluble such as acetals of polyvinyl alcohol and o-sulfo-benzaldehyde, carrageenin (a natural polysulfate of polygalactau), the copolymer of methylvinyl ether and maleic anhydride described in Example V of Moede, US. 2,772,165, issued November 27, 1956, assigned to assignee, etc., anion soaps such as discussed at pages 519-522 of the book Kolloid-Chernische Grundlagen der Textilveredlung by Dr. E. Valko, 1937, salts such as Na SO and NH NO and combinations of various coagulating agents.

The emulsions precipitated, in accordance with the invention, may be silver chloride, silver bromide, silver chlorobromide, silver iodobromide, or composed of mixed silver halides which can be made in the usual manner from soluble halides and soluble silver salts. The emul- 8 sions may be optically sensitized by incorporation of the conventional optical sensitizing dyes.

Various hydrophilic organic colloids such as hydrophilic synthetic polymers may be employed as a binder in these emulsions, being added during the redispersion step which follows the second coagulation. Since some gelatin is already present, a mixed binder system results. Suitable hydrophilic synthetic polymers for use in such a mixed binder system include any of various hydrolysis grades of polyvinyl alcohol, polyvinyl acetals, carboxylated polyvinyl acetals, polyvinyl pyrrolidone, etc.

Some care must be exercised in the choice of that small amount of gelatin which is present during ripening and digestion of the emulsion. However, the much larger amount of gelatin which is added after the second coagulation may be selected from a wide variety of gelatin types including some which are considerably lower in cost than normally used in photographic emulsions.

The emulsions may be coated on any suitable support to make the desired photographic element. Suitable supports include cellulose acetate, other cellulose esters such as cellulose triacetate, cellulose acetate butyrate, cellulose nitrate, cellulose propionate; super polymers, e.g., polyethylene terephthalate, nylon, polyvinyl chloride, poly (vinyl chloride or vinyl acetate) and vinylidene copolymers with vinyl acetate, acrylonitrile, etc.; metal sheets or foils, e.g., aluminum and zinc; and paper including resin-coated and bartya-coated paper; as well as glass plates.

The primary advantage of this invention is that it affords a simple, reliable means of preparing a photographic emulsion when it is desired that said emulsion have a high solids content. This is particularly important for extrusion coating and is, to a lesser extent, advantageous in air knife coating. Another advantage resides in the fact that emulsions prepared according to this invention are not so dependent upon the quality of the gelatin as has been the case heretofore. Only very small quantities of gelatin are present throughout the ripening and digestion phases of emulsion manufacture and the bulk gelatin, which makes up the great majority of the total gelatin present, is added to the emulsion just before coating and therefore contributes relatively little to the sensitometric properties of the coating. Thus, it is feasible to use a relatively inexpensive gelatin for bulking purposes. Also, it is advantageous for one to be able to add types of gelatin during bulking which might bestow certain unique and beneficial properties but which could not have been used during ripening and digestion because of other adverse efifects. Furthermore, by means of this invention it is possible to bulk the emulsion with various hydrophlic synthetic polymers in place of gelatin where these other polymers might have had a detrimental effect had they been present during ripening or digestion.

Still another advantage of a coagulation washing which follows digestion is the removal of excessive reagent employed during the digestion reaction, thus extending the range of applicable compounds, as well as the removal of harmful materials formed during the process of sensitization. The process of this invention also leads to improved aging stability without affecting fresh sensitometric results. This invention is of general application for sensitizing and providing a means of separating chemical sensitization from the other steps of emulsion preparation, thus avoiding many compromises which must be made in conventional systems. Chemical sensitization in the absence of large quantities of binder allows the use of chemicals which, in the presence of the binder, might lose most of their activity. Yet another advantage is the convenience in being able to store emulsions, after digestion, in the form of concentrated emulsion coagulates which, by the process of this invention, have been shown to be stable over long periods of time. Still further advantages will be apparent from the above description of the invention.

What is claimed is:

1. A process for preparing photographic emulsions which comprises, in order, the steps of (a) redispersing a washed coag llum of a precipitated and ripened gelatinosilver halide emulsion in an aqueous solution so that not more than 25% by weight of gelatin, based on the silver halide, is present in the final redispersion resulting from this step (a), (b) adding a photographic emulsion chemical sensitizing agent and digesting the redispersion at a temperature of at least 90 F., (c) coagulating the redispersion, (d) removing the supernatant liquid and (e) redispersing the recoagulated redispersion in an aqueous solution containing a water-permeable colloid of high molecular weight and possessing protective colloid properties.

2. A process as defined in claim 1 wherein the coagulum is a complex of a gelatin emulsion with an anion soap.

3. A process as defined in claim 1 wherein the coagulum is a complex of a gelatin emulsion and a sodium salt of a sulphated monohydric alcohol of 10 to 18 carbon atoms.

4. A process as defined in claim 1 wherein the coagulum is a complex of a gelatin emulsion with a water-soluble, acid-soluble organic polymer of high molecular Weight containing recurring groups taken from the class consisting of oxyacid groups and their alkali metal and ammonium salts.

5. A process as defined in claim 1 wherein the coagulum is a complex of gelatin and a water-soluble, acidsoluble, sodium o-sulfobenzaldehyde polyvinyl acetal containing a plurality of intralinear CH CHOH groups and being both soluble in water and 1% nitric acid to the extent of at least 1% by weight.

6. A process as defined in claim 1 in which, following the removal of supernatant liquid in step (d), the coagulum is Washed with water.

7. A process as defined in claim 1 in which said colloid used in step (e) is gelatin.

8. A process as defined in claim 1 wherein the aqueous solution used during the redispersing step contains gelatin.

References Cited in the file of this patent UNITED STATES PATENTS 1,623,499 Sheppard et al. Apr. 5, 1927 2,401,051 Crouse et a1 May 28, 1946 2,618,556 Hewitson et a1 Nov. 18, 1952 2,728,662 Yutz et a1. Dec. 27, 1955 2,772,165 Moede Nov. 27, 1956 2,787,545 Gates et al. Apr. 2, 1957 2,982,652 De Pauw et a1. May 2, 1961 3,002,741 De Pauw et a1. Sept. 19, 1961 3,046,134 Dann et a1. July 24, 1962 3,067,035 Oberth et a1. Dec. 4, 1962 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 153,594 Qctober 2O 1964 Adolf Eduard Oherth I It is hereby certified-that error appears in the above numbered pat ent requiring correction and that the said Letters Patent should read as corrected belo' v Column 2, line 65, strike out "of"; column 3, line 41, for "o-sulfobanzaldehyde" read 0-sulfobenzaldehyde line 72, for "polyvinly" read polyvinyl column line 43, for "250 g. read 2250 g. line 56, for "sensitiometric" read sensitometric column 5, in the table, heading to the second column, for "Coatingwt. Mg. AgBrq/s. dm." read Coating Wt.,, Mg. AgBr/sq. dma column 9, line 13 before "colloid" insert organic Signed and sealed this 23rd day of March-1965.

(SEAL) Attest:

ERNEST W. SWIDER' EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A PROCESS FOR PREPARING PHOTOGRAPHIC EMULSIONS WHICH COMPRISES, IN ORDER, THE STEPS OF (A) REDISPERSING A WASHED COAGULUM OF A PRECIPITATED AND RIPENED GELATINOSILVER HALIDE EMULSION IN AN AQUEOUS SOLUTION SO THAT NOT MORE THAN 25% BY WEIGHT OF GELATIN, BASED ON THE SILVER HALIDE, IS PRESENT IN THE FINAL REDISPERSION RESULTING FROM THIS STEP (A), (B) ADDING A PHOTOGRAPHIC EMULSION CHEMICAL SENSITIZING AGENT AND DIGESTING THE REDISPERSION AT A TEMPERATURE OF AT LEAST 90*F., (C) COAGULATING THE REDISPERSION, (D) REMOVING THE SUPERNATANT LIQUID AND (E) REDISPERSING THE RECOAGULATED REDISPERSION IN AN AQUEOUS SOLUTION CONTAINING A WATER-PERMABLE COLLOID OF HIGH MOLECULAR WEIGHT AND POSSESSING PROTECTIVE COLLOID PROPERTIES. 